Should I keep my relic gear?

I did hesitate to sell my first relics early in the game, but later you get so many of those that selling or dismantling them is really the only option. You only need one good item for each slot, and the witcher sets you craft are usually the best items you can get for their level.

Dismantling them can make sense if it results in crafting materials you don't have in abundance. You'll have to check for each item what the result of dismantling is. You probably don't need to worry too much, I had no problems crafting all upgrades for one witcher set up to the mastercrafted version, and I almost never dismantled gear. You might need to buy a few materials, but you generally have enough money later in the game that this is not an issue.

Runes provide minor boosts, you won't get much of a benefit from trying to optimize anything monster-specific here.

You can keep the gear in your stash or put it on display in your home Corvo Bianco, if you have the Blood and Wine DLC.

Dismantling it generally results in much lighter gems / dust, that hold value nicely. After you have crafted the Witcher class gear, you'll find that little matches it (let alone bests it).

You can keep the weapons, and optimise them for a specific monster type, but will likely find it inefficient in comparison to a good sword with decent oil applied, against weight-used.